The syntaxonomy of the Ostrya carpinifolia woods in the Southern Alps has been studied by means of phytosociological relevés carried out in the area extended from the Trieste Karst westwards to the Lombard Praealps. Syntaxonomic levels have been defined on the basis of similarity measures obtained by application of formal methods of numerical classification. The ecological conditions of the resulting vegetation types have been discussed. Five associations can be distinguished, all belonging to the alliance Ostryo-Carpinion orientalis (order: Quercetalia pubescenti-petraeae, class. Querco-Fagetea): Ostryo-Quercetum pubescentis, Buglossoido-Ostryetum, Orno-Ostryetum, Melica uniflora-Ostrya carpinifolia association, Seslerio variae-Ostryetum. Two suballiances, essentially of chorological significance, can be distinguished within Ostryo-Carpinion orientalis: Ostryo-Carpinenion orientalis in the Illyric region and Orno-Ostryenion in the Praealps.